Many generic types of self-propelled construction machines have no damping or spring system. This is firstly because springing is disadvantageous to loading procedures due to its yield under lifting and frictional forces, and secondly because provision of a spring system involves high structural outlay which entails not inconsiderable investment and maintenance costs. However, the good driving response, e.g. agility and handling, of such unsprung construction machines are accompanied by a lack of driving comfort, particularly in the working, transport and transfer cycles.
Nevertheless, the time factor is crucial to the cost-effectiveness of such construction machines. Self-propelled construction machines are frequently moved between different building sites at short notice, with the time required for transferring them—i.e. the transfer cycle—playing a decisive role.
However, in unsprung construction machines, an increase in road speed to reduce transfer times is closely linked with the requirements for driving comfort and safety and the permissible stresses for the operator from the point of view of health and safety. If a certain road speed is exceeded, high unwanted pulses and vibrations are recorded which are transmitted to the cab.
In recent years an attempt has therefore been made to find a compromise between the driving behavior and driving comfort of self-propelled, unsprung construction machines, using passive vibration attenuation systems, for example in wheeled loaders. In contrast, active vibration attenuation systems are of no practical significance to structural implementation, due to their complexity and the associated problems.
A hydraulic system in the form of a passive vibration attenuation system for mobile machines fitted with implements is known from DE 42 21 943 C2. In this case it is anticipated that a hydraulic accumulator be used as a load springing system, the hydraulic pipes responsible for raising and lowering the implement being connected between the hoist cylinder and a control valve. It is disclosed that at least one nozzle is provided in conjunction with several directional valves between the load springing system for variable adjustment of the load pressure of the hydraulic accumulator to the respective load pressure of the hoist cylinder, the valves in pilot pipes being operated by manometric switches provided between a pilot sensor and the control valve. In principle, this passive vibration attenuation system uses the yield of the hydraulic accumulator to permit an antiphase movement of the configuration, which itself attenuates the movement of a shovel in relation to the construction machine.
The disadvantage of this solution is that not only the hydraulic accumulator, but also additional directional valves, manometric switches, and nozzles must be provided in the construction machine, automatically entailing higher costs.
So-called suspension systems, which are predominantly used in agricultural tractors, are also known from the state of the art.
This involves combinations of springs and hydraulic dampers in parallel circuits. The damping characteristic is fixed in passive systems (fixed nozzles) and electronically-modifiable in active systems.
The fundamental difference between the passive vibration attenuation system described above and a suspension system lies in the mechanical structure of the moving masses, whereby the suspension system is itself a spring-damper element located between the mass of the vehicle and the individual masses of the wheels and axles, to remove unwanted vibratory movements by dissipation. An invention for the attenuation of movement in construction machines which works on the basis of an electro-hydraulic system for controlling the hoist cylinder is also known from U.S. Pat. No. 5,897,287 A. The purpose of this invention is to ensure a constant pressure in the hoist cylinders. The pressure in the hoist cylinders is permanently monitored and kept constant by means of a pressure sensor, taking the position of the shovel into account, to prevent unwanted lowering of the shovel.
The hydrodynamic valves are a particular disadvantage of this solution. They are necessary for the requisite pressure regulation, but not for attenuating movement in wheel loaders. Experience has shown that excitation/pulses or pulse oscillation generated by the pitching of the loaded shovel can be well compensated in this way, but this solution is unsuitable for cab vibrations.
In conclusion, it must be stated that the passive movement or vibration attenuation system already known from the state of the art is not optimized—or only optimized with restrictions—for changing operating conditions, and that it is only designed for quite specific problems if attenuation of cab vibration is taken into account. Transferring the suspension systems used in agricultural engineering to unsprung construction machines is not possible, for reasons of a permanent connection between the front axle and the front frame. Very high costs also arise from an unjustifiable outlay for highly-dynamic pressure control valves with the use of the active vibration attenuation system already known from U.S. Pat. No. 5,897,287 A.
A device for attenuating movement in self-propelled, unsprung construction machines (e.g. excavators) is known from U.S. Pat. No. 5,832,730. The implement is driven by means of a hydraulic cylinder. The construction machine also has a hydraulic source, a controlled valve for supplying the hydraulic cylinder with hydraulic fluid and a control unit with control software. Two pressure sensors are provided on the boom cylinder, the measurement signals from which are processed as incoming signals by the control software and converted into an acceleration signal, from which a pilot current is determined for the valve as an output variable for a compensating movement by the hydraulic cylinder. This device becomes effective when the implement is operated by the driver, i.e. the driver's control signals are overridden to attenuate movement automatically if unwanted movements occur. This specification does not disclose attenuation of movement during travel, independently of operation of the implement by the driver.